Why is the mail app (Gmail) on my Lumia 830 taking an unusual peroid of time to sync?

The sync settings for new mail content is set for : " based on data usage" which further shows currently syncing: as items arrive. The mails arrive in 15-20 minutes after either the Wifi or Mobile data is switched on. And for example if a mail is sent to my account at 10:00 am, it arrives on time on the desktop version, but is delayed in my phone's mail app. My colleagues with other lumia devices are not facing this problem. All the mails from various accounts get sync within a minute or so after a mail is sent or the data sources are switched on. Please help me with the above mentioned issue.